Processing seismic data using interferometry techniques

ABSTRACT

Method for processing seismic data. In one implementation, the method includes converting a common midpoint (CMP) gather of seismograms into one or more interferogram common midpoint (ICMP) gathers, generating a semblance spectrum for each ICMP gather, stacking the semblance spectrum from each ICMP gather to generate a combined semblance spectrum and deriving a normal moveout (NMO) velocity profile from the combined semblance spectrum.

BACKGROUND

1. Field of the Invention

Implementations of various technologies described herein generallyrelate to seismic data processing.

2. Description of the Related Art

The following descriptions and examples are not admitted to be prior artby virtue of their inclusion within this section.

In a typical seismic survey, a plurality of seismic sources, such asexplosives, vibrators, airguns or the like, may be sequentiallyactivated at or near the surface of the earth to generate energy whichmay propagate into and through the earth. The seismic waves may bereflected back by geological formations within the earth. The resultantseismic wavefield may be sampled by a plurality of seismic sensors, suchas geophones, hydrophones and the like. Each sensor may be configured toacquire seismic data at the sensor's location, normally in the form of aseismogram representing the value of some characteristic of the seismicwavefield against time. A seismogram may also be commonly known as aseismic trace. The acquired seismograms may be transmitted wirelessly orover electrical or optical cables to a recorder system. The recordersystem may then store, analyze, and/or transmit the seismograms. Thisdata may be used to detect the possible presence of hydrocarbons,changes in the subsurface and the like.

Seismograms may contain unwanted signals, or noise, as well as thedesired seismic reflection signals. Unwanted signals may interfere withthe interpretation of the seismic signals and degrade the quality of thesubsurface images obtained by processing the recorded seismograms. Itmay therefore be desirable to suppress or attenuate the unwanted signalthat may be present in the recorded seismograms during processing.Various techniques have been developed to process seismograms in aneffort to amplify the seismic reflection signals and attenuate theunwanted signals, such as semblance spectrum velocity analysis, normalmoveout (NMO) correction, NMO stacking and the like. Other commontechniques used to process seismograms include tools to manipulatetravel times in seismograms. Deconvolution, also referred to as spectraldivision, may be one such tool. After deconvolution, the seismic datamay be recorded according to travel time difference rather than traveltime.

In the field of interferometry, seismograms may be converted intoseismic interferograms by deconvolving two seismograms that have beentransformed from the time domain into the frequency domain. This processcombines the two seismograms into one seismic interferogram thatcontains the difference between the two seismograms, cancelling out allthat may be in common between the seismograms such as unwanted signals.

Typical techniques applied during seismic data processing may notadequately amplify the reflection signal and attenuate the unwantedsignal. Accordingly, improved methods for processing seismic data may bedesirable. Using interferometry techniques combined with typical seismicdata processing techniques may improve various methods for processingseismic data.

SUMMARY

Described herein are implementations of various technologies for amethod for processing seismic data. In one implementation, the methodincludes converting a common midpoint (CMP) gather of seismograms intoone or more interferogram common midpoint (ICMP) gathers, generating asemblance spectrum for each ICMP gather, stacking the semblance spectrumfrom each ICMP gather to generate a combined semblance spectrum andderiving a normal moveout (NMO) velocity profile from the combinedsemblance spectrum.

In another implementation, the method includes deriving a normal moveout(NMO) velocity profile, converting a common midpoint (CMP) gather ofseismograms into one or more interferogram common midpoint (ICMP)gathers, correcting the ICMP gathers using the NMO velocity profile andstacking the NMO corrected ICMP gathers.

Described herein are also implementations of various technologies for acomputer system. In one implementation, the system includes a processorand a memory comprising program instructions executable by the processorto: prepare one or more seismograms into a common midpoint (CMP) gather,convert the CMP gather into one or more interferogram common midpoint(ICMP) gathers, generate a semblance spectrum for each ICMP gather,stack a semblance spectra from the one or more ICMP gathers to generatea combined semblance spectrum, derive a normal moveout (NMO) velocityprofile from the combined semblance spectrum and generate NMO stacks ofseismic data using the NMO velocity profile.

The claimed subject matter is not limited to implementations that solveany or all of the noted disadvantages. Further, the summary section isprovided to introduce a selection of concepts in a simplified form thatare further described below in the detailed description section. Thesummary section is not intended to identify key features or essentialfeatures of the claimed subject matter, nor is it intended to be used tolimit the scope of the claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

Implementations of various technologies will hereafter be described withreference to the accompanying drawings. It should be understood,however, that the accompanying drawings illustrate only the variousimplementations described herein and are not meant to limit the scope ofvarious technologies described herein.

FIG. 1 illustrates a flow diagram of a method to improve thesignal-to-noise ratio in NMO stacked seismic data using interferometryin accordance with implementations of various technologies describedherein.

FIGS. 2A-D illustrate semblance spectrum velocity analysis usingseismograms and seismic interferograms in accordance withimplementations of various technologies described herein.

FIG. 3 illustrates a flow diagram of a method to generate a semblancespectrum using seismic interferograms in accordance with implementationsof various technologies described herein.

FIGS. 4A-D illustrate NMO correction and stacking using seismograms andseismic interferograms in accordance with implementations of varioustechnologies described herein.

FIG. 5 illustrates a flow diagram of a method to generate NMO stacksusing seismic interferograms in accordance with implementations ofvarious technologies described herein.

FIG. 6 illustrates a computer network, into which implementations ofvarious technologies described herein may be implemented.

DETAILED DESCRIPTION

In general, one or more implementations of various technologiesdescribed herein are directed to a method for processing seismic datausing seismic interferometry techniques, which are based on traveltimedifferences. The seismograms may first be sorted into a common midpoint(CMP) gather, which then may be converted to a plurality ofinterferogram common midpoint (ICMP) gathers. After the conversion, asemblance spectrum may be computed for the ICMP gathers. The semblancespectrum may then be used to derive a normal moveout (NMO) velocityprofile, which may then be used to generate NMO stacks using either CMPgathers or ICMP gathers. In one implementation, after the conversion ofCMP gathers to ICMP gathers, NMO stacks may be generated from the ICMPgathers without first computing the semblance spectrum in the seismicinterferogram domain. Various implementations of the method aredescribed in more detail in the following paragraphs.

FIG. 1 illustrates a flow diagram of a method 100 to improve thesignal-to-noise ratio in NMO stacked seismic data using interferometryin accordance with implementations of various technologies describedherein. It should be understood that the operations illustrated in theflow diagram of FIG. 1 are not necessarily limited to being performed bymethod 100. Additionally, it should be understood that while theoperational flow diagram indicates a particular order of execution ofthe operations, in other implementations, the operations might beexecuted in a different order.

At step 110, a common midpoint (CMP) gather may be prepared usingseismograms from any type of seismic survey, such as land or marine. ACMP is a point on the earth's surface that is equidistant from one ormore source and sensor pairs. The CMP on the earth's surface may bedirectly above a seismic reflection event below the earth's surface. Ina typical seismic survey, there may be a plurality of CMPs. Duringprocessing, the survey seismograms may be sorted such that seismogramshaving the same CMP are grouped together. A group of seismograms sharinga CMP is known as a CMP gather. FIG. 2A illustrates a CMP gather whereeach vertical entry represents a seismogram with an amplitude value at agiven time and offset from the CMP. Although various implementations aredescribed herein with reference to a CMP gather, it should be understoodthat in some implementations, the seismograms may be grouped into othertypes of gathers, such as common source, common receiver or commonoffset gathers.

At step 120, one or more of the seismograms in the CMP gather may beselected as reference seismograms. In one implementation, allseismograms in a CMP gather may be selected as reference seismograms. Atstep 130, each seismogram in the CMP gather may be converted from thetime domain to the frequency domain using Fourier transforms.

At step 140, each seismogram of the CMP gather may be deconvolved byeach reference seismogram. For example, each seismogram of the CMPgather may be deconvolved by a first reference seismogram. Eachdeconvolution may generate a seismic interferogram. Hence, for onereference seismogram, as many seismic interferograms may be generated asseismograms in the original CMP gather. All seismic interferogramsgenerated by one reference seismogram may be referred to as aninterferogram CMP (ICMP) gather. Next, each seismogram of the originalCMP gather may be deconvolved by a second reference seismogram togenerate another ICMP gather. Thus, the deconvolution process may yieldas many ICMP gathers as selected reference seismograms and as manyseismic interferograms in each ICMP gather as seismograms in theoriginal CMP gather.

The mathematical computation performed by deconvolution may cause allcommon convolution components between each seismogram and the referenceseismogram to be eliminated. If the seismograms were collected in closetime and proximity, as in a CMP gather, it may be assumed that manyunwanted signals may be common components, such as the source wavelet,common near-surface effects on both the source and receiver side, commonabsorption wavelet, and the like. For example, each seismogram in thefrequency domain may be represented by the following equation:x ₁(f)=w(f)·r ₁(f)  Equation 1.In Equation 1, w(f) represents the unwanted signals and r₁(f) representsthe reflection signal. The reference seismogram may be represented bythe following equation:x _(ref)(f)=w(f)·r _(ref)(f)  Equation 2.In Equation 2, w(f) represents the unwanted signals and r_(ref)(f)represents the reflection signal of the reference seismogram. Theseismogram represented by Equation 1 may be deconvolved with thereference seismogram of Equation 2 to form the following equation:

$\begin{matrix}{\frac{x_{1}(f)}{x_{ref}(f)} = {\frac{w(f)}{w(f)} \cdot {\frac{r_{1}(f)}{r_{ref}(f)}.}}} & {{Equation}\mspace{14mu} 3}\end{matrix}$Because the common unwanted signals w(f) cancel out of Equation 3,common unwanted signals w(f) may be eliminated via the deconvolutionprocess. Thus, converting seismograms to seismic interferograms mayeliminate common unwanted signal and improve the signal-to-noise ratioin the seismic data. It should be noted that the deconvolution processmay also be referred to as spectral division. It should also be notedthat the deconvolution process may be replaced by a correlation process.In one implementation, the seismic interferograms may be created in atime variant manner, for example using sliding time windows.

At step 150, an inverse Fourier transform may be applied to each seismicinterferogram to convert them from the frequency domain back into thetime domain. The resulting traveltime information in a seismicinterferogram may be a relative traveltime in the form of the differenceof the traveltimes of the seismogram and the reference seismogram thatwere deconvolved to produce it. The traveltime may be relative to thereference seismogram used to calculate the ICMP gather. For example, ifan event exists at traveltime t_(a) in a seismogram and at traveltimet_(ref) in the reference seismogram, the seismic interferogram may havethe event at the traveltime difference represented in Equation 4 below.τ=t _(a) −t _(ref)  Equation 4Thus, the traveltime information of the original data may still beencoded in the seismic interferograms such that moveout information maystill be available in the seismic interferograms. FIG. 2B illustrates anICMP gather where each vertical entry represents a seismic interferogramwith an amplitude value at a given relative time and offset from theCMP. The ICMP gather illustrated in FIG. 2B shows the data from the CMPgather illustrated in FIG. 2A in the seismic interferogram domain.

At step 160, a semblance spectrum may be generated for each ICMP gatherproduced in step 140. A semblance spectrum may be a method of velocityanalysis used to obtain the moveout velocity of events. The conventionaluse of semblance spectra for velocity analysis may be modified to beapplied to seismic interferograms instead of seismograms, as describedin FIG. 3 below. The result may be an improved semblance spectratechnique with better unwanted signal suppression.

FIG. 3 illustrates a flow diagram of a method 300 to generate asemblance spectrum using seismic interferograms in accordance withimplementations of various technologies described herein. It should beunderstood that the operations illustrated in the flow diagram of FIG. 3are not necessarily limited to being performed by method 300.Additionally, it should be understood that while the operational flowdiagram indicates a particular order of execution of the operations, inother implementations, the operations might be executed in a differentorder.

At step 310, a zero offset traveltime may be selected. Each ICMP gathermay have a minimum and maximum zero offset value. Any zero offsettraveltime within the minimum and maximum range may be selected. At step320, an NMO velocity may be selected. Each ICMP gather may have aminimum and maximum NMO velocity. Any NMO velocity within the minimumand maximum range may be selected.

At step 330, a semblance value may be computed along the traveltimedifference curve defined by the selected zero offset traveltime and NMOvelocity. In conventional semblance value calculations using seismogramsmay use the following equation:

$\begin{matrix}{{t(h)} = \sqrt{t_{0}^{2} + \frac{h^{2}}{v^{2}}}} & {{Equation}\mspace{14mu} 5}\end{matrix}$

In Equation 5, t₀ represents the zero offset traveltime, v representsthe NMO velocity and h represents the offset from the CMP. Becausetraveltime in seismic interferograms may be represented as thetraveltime difference of Equation 4, Equation 5 may be modified for usein the seismic interferogram domain as shown below.

$\begin{matrix}{{\tau(h)} = {{{t(h)} - {t\left( h_{ref} \right)}} = {\sqrt{t_{0}^{2} + \frac{h^{2}}{v^{2}}} - \sqrt{t_{0}^{2} + \frac{h_{ref}^{2}}{v^{2}}}}}} & {{Equation}\mspace{14mu} 6}\end{matrix}$Because the zero offset traveltime to and the NMO velocity v have beenselected and the offset h and reference offset h_(ref) define theseismic interferogram and are therefore known, Equation 5 may becalculated to obtain a semblance value.

At step 340, the semblance value from step 330 may be mapped into thesemblance spectrum. The semblance spectrum may be a matrix with the zerooffset traveltime along one axis and the NMO velocity on the other axis.Semblance values of substantially one may be recorded as events, whilesemblance values of substantially zero may be recorded as non-events.FIG. 2C illustrates a typical semblance spectrum calculated from thedata in FIG. 2A. In FIG. 2C, semblance values of substantially one maybe recorded as black, while semblance values of substantially zero maybe depicted by white. Dot patterns with increasing density representvalues approaching one.

At step 350, steps 310-340 may be repeated for every combination of zerooffset traveltime and NMO velocity for the ICMP gather. FIG. 2Dillustrates the semblance spectrum that may be computed from the ICMPgather of FIG. 2B. FIGS. 2A and 2B illustrate the same seismic data inthe seismogram and seismic interferogram domains respectively.Consequently, FIGS. 2C and 2D also represent the same seismic data. Inthe conventional semblance spectrum of FIG. 2C, a first event 210 at 0.6seconds may be obscured by unwanted signals in the seismogram. However,in the semblance spectrum of FIG. 2D calculated from seismicinterferograms, the unwanted signal may be substantially suppressed,increasing the signal-to-noise ratio and isolating the first event 210at 0.6 seconds.

Returning to the flow diagram of FIG. 1, in one implementation, thesemblance spectra generated in steps 110-160 may be used in anyconventional manner, such as NMO corrections before stacking, dipmoveout corrections and the like.

Referring now to step 170, one or more semblance spectra produced instep 160 may be stacked to produce a combined semblance spectrum.Stacking may be accomplished by summing the corresponding semblancevalues and dividing the sum by the number of values summed. In oneimplementation, the resulting combined semblance spectrum may be used inany conventional manner, such as the various methods described in theabove paragraph with reference to the combined semblance spectrum.

At step 180, an NMO velocity profile may be derived by interpreting thecombined semblance spectrum of step 170 by conventional methods. An NMOvelocity profile may be used to prescribe corrections necessary to alignevents before seismic data may be stacked. As described above,seismograms in a CMP gather have the same CMP and the same reflectionevent below the earth's surface. A reflection typically arrives first atthe receiver nearest the source. The offset between the source and otherreceivers induces a delay in the arrival time of a reflection event.Therefore, a reflection event may be recorded in a plurality ofseismograms with delays in reflection arrival times. The effect of theseparation between receiver and source on the arrival time of areflection event may be referred to as normal moveout. The NMO velocitymay define the moveout curvature. The NMO velocity profile may then beutilized for any conventional processing methods.

In one implementation, referring to step 190A, the NMO velocity profileof step 180 may be used to perform NMO stacking of the originalseismograms of the CMP gather of step 110. The NMO velocity profile maybe used to correct the seismograms of the CMP gather. The seismograms ofthe CMP gather may then be stacked to produce an approximation to thenon-acquirable zero offset seismogram. The CMP gather may be correctedseparately for each seismic event such that a single CMP gather mayproduce many NMO corrected stacks. For example, FIG. 4A illustrates anNMO corrected CMP gather where each vertical entry represents aseismogram with an amplitude value at a given time and offset from theCMP. FIG. 4B illustrates a stack of NMO corrected seismograms correctedfor an event at 0.7 seconds. Note that while the event at 0.7 secondsmay be amplified, it may still contain unwanted signals as well asreflection signal. A second event at 1.5 seconds was not the focus ofthe correction, yet may appear in FIG. 4B as unwanted signal.

In another implementation, referring to step 190B, the NMO velocityprofile of step 180 may be used to generate NMO stacks using the seismicinterferograms of step 150. FIG. 5 illustrates a flow diagram of amethod 500 to generate NMO stacks using seismic interferograms inaccordance with implementations of various technologies describedherein. It should be understood that the operations illustrated in theflow diagram of FIG. 5 are not necessarily limited to being performed bymethod 500. Additionally, it should be understood that while theoperational flow diagram indicates a particular order of execution ofthe operations, in other implementations, the operations might beexecuted in a different order.

At step 510, one or more ICMP gathers may be NMO corrected using an NMOvelocity profile. Once they are NMO corrected, the ICMP gathers may bereferred to as pseudo CMP gathers because they may be made up of aplurality of pseudo seismograms. Pseudo seismograms may refer to seismicdata presented as a seismogram, with amplitude measured against time,but derived from seismic interferograms. Pseudo seismograms may beproduced from seismic interferograms and an NMO velocity profile usingthe following equation:

$\begin{matrix}{t^{\prime} = {\sqrt{t_{0}^{2} + \frac{h^{2}}{{v\left( t_{0} \right)}^{2}}} - {\sqrt{t_{0}^{2} + \frac{h_{ref}^{2}}{{v\left( t_{0} \right)}^{2}}}.}}} & {{Equation}\mspace{14mu} 7}\end{matrix}$In Equation 7, v(t₀) represents the NMO velocity, t₀ represents the zerooffset traveltime, and h and h_(ref) are known offsets identifying theseismic interferogram. Using Equation 7, a pseudo seismogram withtwo-way traveltime may be generated. FIG. 4C illustrates an NMOcorrected pseudo CMP gather of pseudo seismograms corrected for theevent occurring at 0.7 seconds, where each vertical entry represents apseudo seismogram with an amplitude value at a given time and offset.FIG. 4C represents the pseudo CMP gather of the CMP data used in FIG.4A.

At step 520, each NMO corrected pseudo CMP gather may be stacked togenerate the equivalent of a normal-moveout stack of the originalseismograms. FIG. 4D illustrates a stack of an NMO corrected pseudo CMPgather. FIG. 4D represents the NMO stack of a pseudo CMP gather of thesame data used with reference to FIG. 4B. Note that unwanted signal maybe better attenuated by NMO stacking of pseudo CMP gathers (asillustrated in FIG. 4D) than by conventional NMO stacking of CMP gathers(as illustrated in FIG. 4B). Each ICMP gather may generate a pseudo CMPgather that may be stacked. Such stack of pseudo CMP gather may befurther stacked to produce the equivalent of a conventional CMP stackfrom the same data.

In one implementation, method 500 may be used to generate NMO stacksfrom seismic interferograms without first computing a semblance spectrumfrom seismic interferograms. In this implementation, following step 150,as an alternative to proceeding to step 160, step 190B may be performedusing an NMO velocity profile derived by any method such as computing asemblance spectrum in the seismogram domain.

Returning to the flow diagram of FIG. 1, in one implementation, the NMOstacks produced from ICMP gathers in step 190B may be used in anyconventional manner, such as improving signal to noise ratio, improvingthe seismic image and the like. As mentioned above, the NMO stacksproduced from the ICMP gathers may be further stacked to produce theequivalent of a conventional CMP stack.

FIG. 6 illustrates a computer network 600, into which implementations ofvarious technologies described herein may be implemented. The computernetwork 600 may include a system computer 630, which may be implementedas any conventional personal computer or server. However, those skilledin the art will appreciate that implementations of various technologiesdescribed herein may be practiced in other computer systemconfigurations, including hypertext transfer protocol (HTTP) servers,hand-held devices, multiprocessor systems, microprocessor-based orprogrammable consumer electronics, network PCs, minicomputers, mainframecomputers, and the like.

The system computer 630 may be in communication with disk storagedevices 629, 631, and 633, which may be external hard disk storagedevices. It is contemplated that disk storage devices 629, 631, and 633are conventional hard disk drives, and as such, will be implemented byway of a local area network or by remote access. Of course, while diskstorage devices 629, 631, and 633 are illustrated as separate devices, asingle disk storage device may be used to store any and all of theprogram instructions, measurement data, and results as desired.

In one implementation, seismic data from the receivers may be stored indisk storage device 631. The system computer 630 may retrieve theappropriate data from the disk storage device 631 to process seismicdata according to program instructions that correspond toimplementations of various technologies described herein. The programinstructions may be written in a computer programming language, such asC++, Java and the like. The program instructions may be stored in acomputer-readable medium, such as program disk storage device 633. Suchcomputer-readable media may include computer storage media andcommunication media. Computer storage media may include volatile andnon-volatile, and removable and non-removable media implemented in anymethod or technology for storage of information, such ascomputer-readable instructions, data structures, program modules orother data. Computer storage media may further include RAM, ROM,erasable programmable read-only memory (EPROM), electrically erasableprogrammable read-only memory (EEPROM), flash memory or other solidstate memory technology, CD-ROM, digital versatile disks (DVD), or otheroptical storage, magnetic cassettes, magnetic tape, magnetic diskstorage or other magnetic storage devices, or any other medium which canbe used to store the desired information and which can be accessed bythe computing system 100. Communication media may embody computerreadable instructions, data structures, program modules or other data ina modulated data signal, such as a carrier wave or other transportmechanism and may include any information delivery media. The term“modulated data signal” may mean a signal that has one or more of itscharacteristics set or changed in such a manner as to encode informationin the signal. By way of example, and not limitation, communicationmedia may include wired media such as a wired network or direct-wiredconnection, and wireless media such as acoustic, RF, infrared and otherwireless media. Combinations of the any of the above may also beincluded within the scope of computer readable media.

In one implementation, the system computer 630 may present outputprimarily onto graphics display 627, or alternatively via printer 628.The system computer 630 may store the results of the methods describedabove on disk storage 629, for later use and further analysis. Thekeyboard 626 and the pointing device (e.g., a mouse, trackball, or thelike) 625 may be provided with the system computer 630 to enableinteractive operation.

The system computer 630 may be located at a data center remote from thesurvey region. The system computer 630 may be in communication with thereceivers (either directly or via a recording unit, not shown), toreceive signals indicative of the reflected seismic energy. Thesesignals, after conventional formatting and other initial processing, maybe stored by the system computer 630 as digital data in the disk storage631 for subsequent retrieval and processing in the manner describedabove. While FIG. 6 illustrates the disk storage 631 as directlyconnected to the system computer 630, it is also contemplated that thedisk storage device 631 may be accessible through a local area networkor by remote access. Furthermore, while disk storage devices 629, 631are illustrated as separate devices for storing input seismic data andanalysis results, the disk storage devices 629, 631 may be implementedwithin a single disk drive (either together with or separately fromprogram disk storage device 633), or in any other conventional manner aswill be fully understood by one of skill in the art having reference tothis specification.

While the foregoing is directed to implementations of varioustechnologies described herein, other and further implementations may bedevised without departing from the basic scope thereof, which may bedetermined by the claims that follow. Although the subject matter hasbeen described in language specific to structural features and/ormethodological acts, it is to be understood that the subject matterdefined in the appended claims is not necessarily limited to thespecific features or acts described above. Rather, the specific featuresand acts described above are disclosed as example forms of implementingthe claims.

1. A method for processing seismic data, comprising: deriving a normalmoveout (NMO) velocity profile; converting a common midpoint (CMP)gather of seismograms into one or more interferogram common midpoint(ICMP) gathers; correcting the ICMP gathers using the NMO velocityprofile; and stacking the NMO corrected ICMP gathers.
 2. The method ofclaim 1, wherein the NMO velocity profile is derived by computing asemblance spectrum in a seismogram domain.
 3. The method of claim 1,wherein the NMO velocity profile is derived by computing a semblancespectrum in a seismic interferogram domain.
 4. The method of claim 1,wherein converting the CMP gather into the ICMP gathers comprises:selecting one or more seismograms as reference seismograms; anddeconvolving each seismogram in the CMP gather with each referenceseismogram.
 5. The method of claim 1, wherein converting the CMP gatherinto the ICMP gathers comprises: selecting one or more seismograms asreference seismograms; applying a Fourier transform to each seismogram;deconvolving each seismogram in the CMP gather with each referenceseismogram to generate the ICMP gathers; and applying an inverse Fouriertransform to each seismic interferogram.
 6. A computer system forgeophysical exploration, comprising: a processor; and a memory, whereinthe memory comprising program instructions executable by the processorto: derive a normal moveout (NMO) velocity profile; convert a commonmidpoint (CMP) gather of seismograms into one or more interferogramcommon midpoint (ICMP) gathers; correct the ICMP gathers using the NMOvelocity profile; and stack the NMO corrected ICMP gathers.
 7. Thecomputer system of claim 6, wherein the NMO velocity profile is derivedby computing a semblance spectrum in a seismogram domain.
 8. Thecomputer system of claim 6, wherein the NMO velocity profile is derivedby computing a semblance spectrum in a seismic interferogram domain. 9.The computer system of claim 6, wherein converting the CMP gather intothe ICMP gathers comprises: selecting one or more seismograms asreference seismograms; and deconvolving each seismogram in the CMPgather with each reference seismogram.
 10. The computer system of claim6, wherein converting the CMP gather into the ICMP gathers comprises:selecting one or more seismograms as reference seismograms; applying aFourier transform to each seismogram; deconvolving each seismogram inthe CMP gather with each reference seismogram to generate the ICMPgathers; and applying an inverse Fourier transform to each seismicinterferogram.